1. Field of the Invention
The present invention generally relates to the molding of thermoplastic materials and more particularly to apparatus and methods of mounting thermoplastic materials on stringed media.
2. Prior Art
There are several classes of processes which are disclosed by the prior art for mounting thermoplastic materials on a supporting medium. The processes are generally described as compression molding, injection molding or casting. Conventional compression molding and injection molding processes are generally carried out by subjecting the thermoplastic material and the supporting media to pressure above 1500 pounds per square inch and at temperatures in excess of 360.degree. F. Depending upon the selection of plastic material and the shape of the mold, molding pressures could be in the order of 10,000 pounds per square inch. The use of high molding pressures is encountered in transfer molding which is an analog of compression and injection molding.
Casting is a typical method utilized in the electronics industry for encapsulating or otherwise embedding circuits or other media within a protective covering. The casting method is expensive and is not applicable to the mass production application to which the present invention is applied. One of the problems inherent in casting is the typical required step of curing the plastic material at elevated periods for periods up to or greater than 12 hours, the exact cure time being dependent upon the temperature, mold shape and the selection of material.
The present invention is directed toward an application whereby thermoplastic bodies are mounted upon a supporting substrate to which typically takes the form of a string or other similar structure. One of the processes disclosed in the prior art for encapsulating a supporting media utilizes a two-piece mold which provides access channels for the supporting medium which in this case is typically a wire or other like structure. The thermoplastic material is enclosed within the mold and the mold raised to the gel temperature. The thermoplastic material is caused to flow around the supporting medium as well as any object to be encapsulated.
The present invention is directed toward a mass production operation whereby a plurality of stringlike structures are passed through the mold, thermoplastic objects being securely fixed to the string in sequential operations. None of the processes described in the prior art exhibit the necessary flexibility to permit execution of the stated operation. The problems inherent in those systems disclosed in the prior art are substantially solved through the use of injecting a heated thermoplastic runner into a bifurcated mold, each half of the mold containing complementary cavities to define the object to be mounted. Prior to injection of the thermoplastic material, the supporting medium is aligned across the cavity, the supporting medium itself being used to prevent flow from one cavity to the other. Subrunners of the thermoplastic material provide for injection into all cavities in a manner which facilitates removal of the molded material subsequent to cooling.